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base and derived unit of the International System of units SI

quantity unit symbol definition
length meter m the metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of second.
mass kilogram kg it is equal to the mass of the international prototype of the kilogram
time second s The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
electric current ampere A The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10 -7 newton per metre of length.
kelvin K The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
amount of substance mole mol The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; This value corresponds to the number of Avogadro
luminous intensity candela cd The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540*10 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
SI supplementary unit
plane angle radian rad angle to the center of a circumference that subtends an arc of equal length to the beam. 1rad =180°/p
solid angle steradian sr angle that on a sphere with center in the apex of the angle intercepts one cap of equal area to that one of a having square equal side to the beam of the same sphere.

unit defined independently to the base SI units

quantity unit symbol definition
mass unified atomic mass unit u the unit of atomic mass is equal to 1/12 of the mass of an atom of the nuclide 12C
energy electronvolt eV the elettronvolt is the kinetic energy acquired by an electron in passing through a potential difference of 1V in vacuum

1 u = 1.6605402*10
-27 kg
1 eV = 1.60217733*10
-19 joule

derived units of the SI
in alphabetical order

This is a list of the definitions that they have represented the material of the fundamental quantities of the International System. From the observation of the preceding definitions one note the tendency to duty release the units of fundamental measure from the materials prototype, for associate it to physical phenomenons. The advantages of this work are a better and reproducible believer, in as the Universal Constants are known with least tolerances, and besides the prototype are not subjects to environmental alterations.

METER: in 1799 it was determined from the length of a bar of platinum, equal to 1/ 40 000 000 of the length of the terrestrial meridian; in the 1875 is reported to the distance of the two nick mark on the sample bar of platinum (90%) iridium (10%), preserved to constant temperature (0°C) in the International Office of Weights and Measures of Sèvres from Paris; in the 1960 had represented from 1650763.73 lengths of wave, in the vacuum, of the corresponding radiation to the transition between the levels 2p10 5d5 of the atom of the krypton-86; it in the 1983 is that currently in use.

: coincident with the mass of 1 dm3 of water distilled to the temperature of 4°C.

SECOND: before the 1960 it was represented from the 86400a part of the solar mean day; after the 1960 it was determined of the fraction of the tropic year to the date of 1900; nel 1967 it is that one currently in use.

CANDELA: before 1982 it was defined from 1/60 of the emitted luminous intensity from the integral radiator (black body) to the temperature of solidification of the platinum (2045 K°), in perpendicular direction to the hole of outlet of the radiation from the radiator, being the hole area of equal to 1 cm2; after 1982 it is that one currently in use.

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